Reactivity of Isocyanate Radical with Formic Acid in the Tropospheric and Outer Space Regions: A DFT and MP2 Investigation in Gaseous Phase

Gas-phase formic acid exists primarily as a cyclic dimer. The mechanism of dimerization has been traditionally considered to be a synchronous process; however, recent experimental findings suggest a possible alternative mechanism by which two formic acid monomers proceed through an acyclic dimer to the cyclic dimer in a stepwise process. To investigate this newly proposed process of dimerization in formic acid, density functional theory and second-order Møller−Plesset perturbation theory (MP2) have been used to optimize cis and trans monomers of formic acid, the acyclic and cyclic dimers, and the acyclic and cyclic transition states between minima. Single-point energies of the trans monomer, dimer minima, and transition states at the MP2/TZ2P+diff optimized geometries were computed at the coupled-cluster level of theory including singles and doubles with perturbatively applied triple excitations [CCSD(T)] with an aug‘-cc-pVTZ basis set to obtain an accurate determination of energy barriers and dissociation energies. A counterpoise correction was performed to determine an estimate of the basis set superposition error in computing relative energies. The explicitly correlated MP2 method of Kutzelnigg and Klopper (MP2-R12) was used to provide an independent means for obtaining the MP2 one-particle limit. The cyclic minimum is predicted to be 6.3 kcal/mol more stable than the acyclic minimum, and the barrier to double proton transfer is 7.1 kcal/mol.

Context.The isomerism of molecules in the interstellar medium and the mechanisms behind it are essential questions in the chemistry of organic molecules in space. In the particular case of simple formic and thioformic acids, the low temperatures found in molecular clouds indicate that cis-trans isomerization in the gas-phase must be impeded. Reactions taking place on top of interstellar dust grains may explain the isomer interconversion at low temperatures.Aims.We studied the isomerization processes of formic and thioformic acid that are likely to take place on the surface of interstellar dust grains after being initiated by H abstraction reactions. Similarly, deuterium enrichment of the acids can occur by the same mechanism. Our objective is to shed light on both topics to expand our understanding of the key precursors of organic molecules in space.Methods.We determined the rate constants for the H abstraction reactions as well as the binding energies for the acids on water ice using ab initio calculations and the instanton method for calculating the rate constants, including quantum tunneling. In addition, we tested the viability of the deuteration of formic acid with tailored experiments and looked for it on the L1544 source.Results.For formic acid, there is a clear dependence of the H abstraction reactions on the isomer of the reactant, with rate constants at ~50 K that differ by five orders of magnitude. Correspondingly, we did not observe the trans-cis reaction in our experiments. In the case of thioformic acid, a very similar cis-trans reactivity is found for abstraction reactions at the thiol (-SH) group in contrast to a preferential reactivity that is found when abstractions take place at the -CH moiety. We found comparable binding energies for both isomers with average binding energies of around −6200 and −3100 K for formic and thioformic acid, respectively. Our binding energy calculations show that the reactions are precluded for specific orientations, affecting the overall isomerization rate. For H abstractions initiated by deuterium atoms, we found very similar trends, with kinetic isotope effects varying in most cases between 13 and 20.Conclusions.Our results support the cis-trans interconversion of cis-formic acid on dust grains, suggesting that such an acid should not withstand the conditions found on these objects. On the other hand, the trans isomer is very resilient. Both isomers of thioformic acid are much more reactive. A non-trivial chemistry is behind the apparent excess of its trans isomer that is found in cold molecular clouds and star-forming regions due to a subtle combination of preferential reactivity and binding with the surface. In light of our results, all the deuterated counterparts of thioformic acid are viable molecules to be present on the ISM. In contrast, only the trans isomer of deuterated formic acid is expected, for which we provide upper bounds of detection. Given the mechanisms presented in this paper, other mechanisms must be at play to explain the tiny fraction of cis-formic acid observed in interstellar cold environments, as well as the current trans-DCOOH and trans-HCOOD abundances in hot-corinos.

The Cl + HCOOH reaction is important in the atmosphere, as the chlorine (Cl) atom is an important oxidant, especially in the marine boundary layer, and formic acid (HCOOH) is one of the most abundant organic acids in the troposphere. The reaction surfaces of the two H abstraction channels were computed by second-order unrestricted Møller–Plesset perturbation theory (UMP2) and density functional theory (DFT) calculations. Relative electronic energies were improved to the RCCSD(T)/CBS and UCCSD(T)-F12/CBS levels. The barrier of the C–H hydrogen abstraction channel was found to be lower by about 10 kcal mol−1. Rate coefficients (k) of this channel were calculated at different temperatures at various variational transition state theory (VTST) levels including tunnelling. For single-level direct dynamics VTST calculations, the computed k (2.5 × 10−13 cm3 molecule−1 s−1) using the BMK (Boese and Martin meta hybrid) functional at the highest level (ICVT/SCT) agrees the best with experimental values at 298 K (1.8 and 2.0 × 10−13 cm3 molecule−1 s−1). For dual-level direct dynamics calculations (RCCSD(T)/CBS//MP2 MEP), an adjusted barrier height of 3.1 kcal mol−1 is required to match the ICVT/SCT k with the experimental values. The computed rate coefficients of the Cl + HCOOH reaction is reported for the first time with a temperature range of 200–1500 K. The implications of the results obtained to atmospheric chemistry are discussed.

Boronic acids react with compounds containing 1,2- or 1,3-diols to form five- or six-membered cyclic boronate esters, respectively, although many factors that influence these reactions are not well understood. In the present study, density functional theory and second-order Møller−Plesset (MP2) perturbation theory were employed to examine the mechanism in which a primary aliphatic amine acts as an internal Lewis base to catalyze the formation of a boron−oxygen−carbon linkage in the methanolysis of H2N−CH2−CHCH−B(OH)2 to afford H2N−CH2−CHCH−B(OH)(OCH3); solvent effects were assessed using the polarized continuum model and explicit water molecules. In vacuo, the lowest-energy conformer of H2N−CH2−CHCH−B(OH)2 was a seven-membered, hydrogen-bonded ring structure in which the boronic acid moiety had a planar, trigonal geometry. The catalytic role of the primary amine group in the methanolysis of H2N−CH2−CHCH−B(OH)2 results from facilitation of a proton transfer from an intermolecular B−O dative-bonded adduct between methanol and this boronic acid, rather than from the formation of an intramolecular B−N dative bond. In the absence of amine catalysis, transition states for the rate-determining proton-transfer step in this methanolysis are 12.8−17.3 kcal/mol higher in energy. In the reaction field of water, a five-membered B−N dative-bonded ring conformer of H2N−CH2−CHCH−B(OH)2 was lowest in energy at the MP2 level, but hydrated zwitterionic structures also appear to play an important role in this complex aminoboronic acid/methanol association and ether formation. In contrast to the PBE1PBE functional, B3LYP gave anomalous results for some steps in the methanolysis when compared with those from the more robust, albeit expensive, ab initio MP2 method.

The embedded-cluster technique is used to simulate the local electronic structure of transition-metal impurities in . The description of the central defect cluster employs an ab initio SCF-MO approach. The quantum cluster consists of 21 ions. Outer crystal regions are modelled on the basis of a shell-model representation. In all cases defect-induced lattice relaxations have been consistently included. Our results, demonstrated for , concern optical transitions, Jahn - Teller effects and questions related to the stability of this defect. The computational level of our ab initio calculations corresponds to Hartree - Fock theory (HF) and the configuration interaction (CI). Additionally, Møller - Plesset perturbation theory and density functional theory have been applied to investigate charge-transfer transitions.

The hydrogen abstraction reactions: a multireference Møller–Plesset perturbation (MRMP) theory study

It remains an extreme challenge to activate thermodynamically unfavorable, chemically inert methane molecules under mild conditions. Herein, we report a molecular-like nickel-thiolate hexameric clu...

In the present work, adsorption and dif fusion of oxygen (O) atom on uranium dinitride (UN2) is studied to map out the preferential UN2(100) surface site. The first principle method based on density functional theory (DFT) within the generalized gradient approximation PBE and the covariant version energy functional PBE + U correction were used. The supercell approach and a coverage dependence of the adsorption structures and energetic were studied in detail for several monolayers’ (ML) range. Potential energy surfaces (PES) corresponding to the interaction between O atom and UN2(100) on surface and subsurface for several sites and layers (Top U and Top N slabs) were calculated and favorable sites were identified with their maxima energy stable positions, which were then analyzed. For all positions, the PES show the same system behavior, when the O atom is sufficiently far from the UN2 surface, and the energy of the system tends to the sum of free UN2 slab and free oxygen atom energies. In return, when the distances decrease, strong interactions appear with presence of important potential wells. Calculation results showed that favored on-surface site for O atom adsorption were found to be near the bridge one for the UN (Top U slab) corresponding to five layers, uranium terminated and top one for (Top N slab) corresponding to six layers nitrogen terminated, the maximum system energy is situated at a position of about 1.2 and 1.5 A from the surface for the two layers types calculations respectively. For subsurface results, only Top N presents a favorable incorporation site at the hollow position and the penetration of O atom is about −0.5 A from the surface. DFT + U study confirms all the results obtained by DFT calculations; that is, the maxima site positions for oxygen atom and the adhesion energy values per atom are of the same order of magnitudes. The adsorption energy per oxygen atom and the mean distance from the top surface gradually decrease with the coverage of O atoms for both on-surface cases, Top U and Top N slabs, with oxygen occupying the favorable site. For the Top N slab hollow site, the incorporation of oxygen through the surface becomes effective from a coverage of 3/8 ML with an encrustation of about −0.3 A.

Photocatalytic Methane Conversion: Insight into the Mechanism of C(sp ³ )–H Bond Activation

A performance study of density functional theory with empirical dispersion corrections and spin-component scaled second-order Møller−Plesset perturbation theory on adsorbate–zeolite interactions

The enhanced role of formic acid on sulfuric acid-ammonia-driven nucleation in forest regions and polluted city areas.

Theoretical design of nitrogen-rich cages for energetic materials

Density functional theory (DFT) calculations are applied to study the structure and bonding properties of groups 3–7 transition metal oxide clusters M x=1–3O and Scx=4–6O with 2y − nx + q = 1, in which n is the number of metal valence electrons and q is the charge number. These clusters include MO2, M 2O3 +, M 2O4 −, and M 3O5 (M = Sc, Y, La); MO2 +, MO3 −, M 2O4 +, M 2O5 −, M 3O6 +, and M 3O7 − (M = Ti, Zr, Hf), and so on. The obtained lowest energy structures of most of these clusters are with character of oxygen-centered radical (O·). That is, the clusters contain oxygen atom(s) with the unpaired electron being localized on the 2p orbital(s). Chromium and manganese oxide clusters (except CrO4 −) do not contain O· with the adopted DFT methods. The binding energies of the radical oxygen with the clusters are also calculated. The DFT results are supported by available experimental investigations and predict that a lot of other transition metal oxide clusters including those with mixed-metals (such as TiVO5 and CrVO6) may have high oxidative reactivity that has not been experimentally identified. The chemical structures of radical oxygen over V2O5/SiO2 and MoO3/SiO2 catalysts are suggested and the balance between high reactivity and low concentration of the radical oxygen in condensed phase catalysis is discussed.

Water increases the photocatalytic oxidation (PCO) and decomposition (PCD) rates of formic acid on TiO2. To identify possible electronic origins for the rate increases, the effects of adsorption of combinations of key adsorbates (hydrogen, hydroxyl, water, formic acid, formate) on anatase TiO2(101) were investigated using density functional theory. The adsorption site and strength of bonds formed play key roles in altering the electronic structure to affect reactivity. Adsorption of hydrogen and water through the twofold-coordinated oxygen (2c-O) atom decreased the reducing power of the surface, and adsorption of hydroxyls through the fivefold-coordinated titanium (5c-Ti) atom increased the reducing power. The adsorption of water, formic acid, and monodentate and bidentate formate through the 2c-O and 5c-Ti atoms had varying effects on the positions of the valence and conduction band edges, depending on the strength of the bonds formed. Water coadsorption decreased the strength of the bonds between adsorbates and surface atoms, thus reducing the adsorbate’s effects. For monodentate formate, water coadsorption increased the reduction potential of the TiO2 surface, consistent with an increase in the photocatalytic reaction rate through a decrease in electron−hole recombination.

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